Actomyosin interaction and mechanical performance modulation in skeletal muscle

Muscle contraction involves the sliding of the myosin-containing thick filaments past the actin-containing thin filaments and the ATP hydrolysis by this actomyosin complex provides energy for contractile process. The force actively generated during contraction is transmitted to the end of the contractile unit through thin filaments, the contribution of titin, a sarcomere scaffold protein, is unclear. Our attention was focused on elastic and mechanic properties of single myofilaments in rigor condition, absence of ATP. Although earlier observations provided strong evidence that filament are rigid, recent results conflict with this proposal: thin and thick filaments are extensible during contraction. Thus, the thin filament properties are crucial parameters for the mechanism contraction. We used laser trap to pull actin filaments and S1-(myosin subfragment 1) decorated actin filaments. Since acto-myosin overlap region length is sliding related, and only a fraction of myosin heads is attached to actin, we tested several S1-decoration percentages, from 20 to 100%. We observed the strain due to the applied stress and calculated Young's modulus and filament elongation. The elastic responses to applied forces decrease when S1-decoration % increase. The maxima elongations decrease from 4.5% for F-actin to 3.4%, 1.9% and 0.5% for 20% 40% and 100% S1-decorated F-actin. We also measured the maximum strain supported by filaments and found a lineard relationship between S1-decoration % and applied tensile strength: forces are twice for 40% and more than 4 times for 100%. These results prove that crossbridges formations influence actin filaments stability, by strengthening the monomer-monomer interaction, and contribute to unload contraction force. we are aware that our observations are not exaustive, and our future aim is to understand the role played by actin associated protein.